1. Field of the Invention
The present invention relates to a gas concentration detecting apparatus and method and a manufacturing method for the apparatus. The apparatus and method uses a gas concentration sensor for outputting a current signal corresponding to a gas concentration of gas to be detected when a voltage is applied. The apparatus and method is, for example, embodied in a gas concentration detecting apparatus applied to a gas concentration feedback control system of an engine employed in a vehicle.
2. Related Art
For application to a vehicle, a gas concentration detecting apparatus using a gas concentration sensor is proposed. As one example, an air-fuel ratio detecting apparatus using an air-fuel ratio sensor is known.
In an air-fuel ratio control of an engine mounted on a vehicle in recent years, for example, there is a demand for improved control accuracy and a demand for a transition to lean-burn. In order to respond to these demands, a linear air-fuel ratio sensor for detecting the air-fuel ratio of air-fuel mixture supplied to the engine (concentration of oxygen in exhausted gas) linearly over a wide zone and an air-fuel ratio detecting apparatus using the sensor are implemented. As such an air-fuel ratio sensor, for example, in an air-fuel ratio sensor of a limit current type, the zone for detecting a limit current is shifted in accordance with the air-fuel ratio (concentration of oxygen) at that time as is generally known.
The air-fuel ratio sensor of a limit current type has output characteristics in which the farther the air-fuel ratio moves to the lean zone, the more the zone for detecting a limit-current is shifted to the positive-voltage side. The farther the air-fuel ratio moves to the rich side, the more the zone for detecting a limit current is shifted to the negative-voltage side. Consequently, if the applied voltage is held set at a fixed value when the air-fuel ratio changes, it would be impossible to detect an air-fuel ratio accurately by using the zone for detecting a limit current. In a conventional air-fuel ratio detecting apparatus, therefore, the voltage applied to the sensor is varied in accordance with the air-fuel ratio at each time, that is, the sensor current (for example, Japanese Patent Laid-Open Nos. Sho-61-237047 and Sho-61-280560). In this case, the applied voltage is controlled on the basis of an application voltage characteristic line Lx in FIG. 3. By controlling the applied voltage in this way, a desired sensor current (limit current) can be always detected.
The circuit construction of an air-fuel ratio detecting apparatus which is general conventionally implemented is shown in FIG. 22. In the diagram, a reference voltage Va generated by a reference voltage circuit 84 is applied to one terminal 82 of an air-fuel ratio sensor 81 and an instruction voltage Vb outputted from a D/A converter 87 is applied to the other terminal 83. The instruction voltage Vb is variably controlled by a CPU (not shown) in accordance with an air-fuel ratio at each time. The circuit construction will be briefly described. The predetermined reference voltage Va generated by the reference voltage circuit 84 is amplified by an amplification circuit 85. The same voltage Va as the reference voltage Va from the reference voltage circuit 84 is applied to one terminal 82 of the air-fuel ratio sensor 81. The instruction voltage Vb outputted form the D/A converter 87 is amplified by an amplification circuit 86. The same voltage vb as the instruction voltage Vb is applied to the other terminal 83 of the air-fuel ratio sensor 81.
The linear type air-fuel ratio sensor 81 conducts a sensor current according to the air-fuel ratio. An A/F output indicative of an air-fuel ratio is therefore detected as an electromotive voltage Vc of a current detection resistor 88 for detecting the sensor current (air-fuel ratio), not a sensor terminal voltage as the predetermined reference voltage Va. In this case, the electromotive voltage Vc is outputted via a voltage follower 89. FIG. 23 is a graph showing a characteristic of an output voltage (A/F value) of each air-fuel ratio. According to the diagram, when the air-fuel ratio is shifted to the lean side, the electromotive voltage Vc is shifted to the positive side with respect to the reference voltage Va. When the air-fuel ratio is shifted to the rich side, the electromotive voltage Vc is shifted to the negative side with respect to the reference voltage Va. A Vc signal (A/F value) obtained in this manner is transmitted from the voltage follower 89 to an A/D converter 90. After A/D-converted by the A/D converter 90, the resultant signal is used for the air-fuel ratio F/B control in a CPU 91 for engine control.
In the air-fuel ratio detecting apparatus having the above configuration, the input voltage range of the A/D converter 90 for receiving the voltage signal (A/F value) is limited to a predetermined range of, for example, "0 to 5V". For instance, in case of using an 8-bit A/D converter, the input voltage range of "0 to 5V" is divided into 256 and the A/F value is read. Specifically, when the air-fuel ratio detection range is set to a zone (A/F=12 to 18) near the stoichiometric ratio in order to perform a stoichiometric control in which the stoichiometric ratio (A/F=14.7) is used as a target air-fuel ratio, the electromotive voltage Vc is outputted in the range of "0 to 5V" by using the current detection resistor 88 in FIG. 22. In this instance, the voltage value per unit A/F (every "1" of the interval of A/F) is "0.833V" and the A/F value is divided into 42 per unit A/F and detected.
On the contrary, for example, in a case where the air-fuel ratio detection range is expanded to A/F=12 to 25 in order to realize lean-burn control, when the air-fuel ratio detecting range is kept set to a range of "0 to 5V", the voltage value per unit A/F is "0.384V" and the A/F value is detected by being divided into 19 per unit A/F. That is, it denotes that the detection accuracy of the air-fuel ratio at the time of the lean-burn control is lower than the detection accuracy of the air-fuel ratio at the time of the stoichiometric control (the higher the voltage value per unit A/F is, the higher the detection accuracy of the air-fuel ratio is). As a result, for example, in the air-fuel ratio control system in which both of the stoichiometric control and the lean-burn control are executed, a problem is caused such that the detection accuracy of the air-fuel ratio near the stoichiometric ratio deteriorates in order to assure the detection accuracy of the air-fuel ratio at the time of lean-burn control.
It is to be noted that the problem of degradation in a detection accuracy of the sensor occurs not only in the air-fuel ratio detecting apparatus but also in all of gas concentration detecting apparatuses which use a gas concentration sensor for producing a current signal in accordance with the gas concentration to be detected and are constructed to detect the gas concentration from a detection result of the sensor.